Chapter 4 Flashcards
(25 cards)
why is strength not a meaningful descriptor of a material’s survival in the mouth/
it is not a reliable property for estimating the survival probabilities over time of prostheses made of brittle material because strength increases with specimen size and stressing rate, decreases with the number of stress cycles, and is strongly a ected by surface processing damage; strength is not a true property of the material
what ar ethe types of stresses in dental materaisl?
tensile, compressive, shear stress
define the strength of a mateiral
the average level of stress at which it exhibits a certain degree of initial plastic deformation (yield strength) or at which fracture occurs (ultimate strength) in test specimens of the same shape and size
what factors is strength dependent on?
(1) stressing rate, (2) shape of the test specimen, (3) size of the specimen, (4) surface nish (which controls the relative size and number of surface aws), (5) number of stressing cycles, and (5) environment in which the material is tested
what is burnishing?
Burnishing of a cast metal margin is a process sometimes used to reduce the width of a gap between the crown margin and the tooth surface; However, because elastic deformation has also occurred, the margin will spring back as elastic strain decreases during the decrease in pressure. us, burnishing the margin will close the gap only to the extent of the plastic deformation (strain) that is induced during burnishing
what is strength?
In a general sense, strength is the ability of the prosthesis to resist induced stress without fracture or permanent deformation (plastic strain).
Although strength is an important factor, it is not a reliable property for estimating the survival probabilities over time of prostheses made of brittle material because strength increases with specimen size and stressing rate, decreases with the number of stress cycles, and is strongly a ected by surface processing damage–so it is NOT A TRUE PROPERTY OF A MATERIAL
what is fracture toughness?
describes the resistance to crack propagation of brittle materials–>is better to refer to this than strength
why do restorations/dental prosthetics fracture after a few years of service?
mastication forces (loading and unloaded propagates minute flaws–>become microcracks via cyclic fatigue
the failure potential under applied force is related to the mechanical props and microsctructure of teh prothestic material
define mechanical properties
the measured responses, both elastic (reversible upon force reduction) and plastic (irreversible or nonelastic), of materials under an applied force, distribution of forces, or pressure–measured in units of stress and/or strain
The stressing rate is also of importance since the strength of brittle materials increase with an increase in the rate at which stress is induced within their structures
ye
How can two different compressive forces applied to the same ceramic crown produce different stresses within the crown surface?
coming soon
what is stress distribution/gradient?
force or pressure exerted on ELASTIC solid–>the stress induced near the surface decreases with distance from the loading point and increases as the supporting surface is approached…so stress is lowest in the middle of the object?
Except for certain exural situations, such as four-point exure, and certain nonuniform object shapes, stress typically decreases as a function of distance from the area of the applied force or applied pressure. us, stress distributions in an elastic solid are rarely uniform or constant;
however, to determine mech. prop. we assume stress is uniform
what is rupturing of interatomic bonds?
the force exerted on an object is so high that the atoms moves so close together–>bonds rupture bc atoms do not like to be so close, so they are trying to break away?
Dental restorations should be designed such that permanent displacement of atoms or rupture of inter- atomic bonds does not occur except possibly at surface areas where normal wear may occur.
displacement of atoms from their eq. position due to an exerted force is related to yielding or plastic deformation
ye
what is strength and on what is it dependent?
strength of a material is de ned as the average level of stress at which it exhibits a certain degree of initial plastic deformation (yield strength) or at which fracture occurs (ultimate strength) in specimens of the SAME SHAPE AND SIZE–remember, strength is not an inherent property
(1) stressing rate, (2) shape of the test specimen, (3) size of the specimen, (4) surface nish (which controls the relative size and number of surface flaws), (5) number of stressing cycles, and (6) environment in which the material is tested; HOWEVER if large flaws exist or stress conc. areas exist bc of improper design this greatly reduces the clinical strength of brittle materials (will fracture at a much lower force because because the localized stress exceeds the strength of the material at the critical location of the flaw)
elastic deformation is reversible (e.g. ceramic crowing elastically deforming during chewing) but plastic deformation occurs when the proportional limit (elastic stress) is exceed and atoms are displaced (plastic deformation) or bonds rupture (bond rupture = cracking of a prosthetic?…cracking =/= plastic deformation?)
ye
how is newton’s 3rd law related to stress?
for every action there is an equal and opposite reaction; so when stress from the outside is exerted on an object, the stress produced within the solid material is equal to the applied force divided by the area over which it acts.
A tensile force produces tensile stress, a compressive force produces compressive stress, and a shear force produces shear stress; a bend force can produce all three stresses but normally a fracture occurs due to the tensile stress
compare and contrast tensile and ductile
tensile force = pulling force on a material to the point that it will break/fracture
ductile force = pulling force that it does not break but rather plastically deforms
does strain = deformation?????
no (i don’t think so at least, not always) ehhhhhhh
The tensile stress (σ), by de nition, is the tensile force per unit area perpen- dicular to the force direction:
σ=F/A, in Pa or MPa
F in Newtons, A in m^2
what is lbf?
pound-force–> equal to 1 pound multiplied by the standard acceleration of gravity on earth (9.80665 m/s2). (not SI)
eq’n for strain?
(ε) is the change in length, Δl, per unit original length, Lo
ε = ΔL/Lo (unitless/in percent (i.e. percent change)) but can be expressed in m/m or cm/m
Why is the maximum elastic strain of a cast alloy used for an inlay or crown an important factor in burnishing a margin?
answer coming soon
describe the types of strain
elastic–>reversible
plastic–>irreversible; deformation/strain does not decrease when force is removed
elastic and plastic–>deformed past the elastic limit into the plastic deformation region; only elastic strain can be relieved; us, when an adjust- ment is made by bending an orthodontic wire, a margin of a metal crown, or a denture clasp, the plastic strain is perma- nent but the wire, margin, or clasp springs back a certain amount as elastic strain recovery occurs.
viscoelastic–>viscoelas. materials deform by exhibiting both viscous and elastic chars.; elastric strain as normal but viscous–>rearrangement of atoms or ‘cules in an amorphous material
